1. Technical Field
The present invention relates to an antenna apparatus including an improved frequency separator using frequency-selective reflecting surfaces (FSRSs).
2. Description of the Prior Art
In satellite communication, an increase in communication capacity necessitates the common use of a single reflector by two or more frequencies. In order that a common reflector can be used by a plurality of frequencies, beams of different frequencies transmitted from a plurality of electromagnetic horns to the reflector have to be composed, or beams of different frequencies reflected from the reflector to the plurality of electromagnetic horns have to be separated. It is known that this objective can be achieved by arranging, in the path of electromagnetic beams propagating through free space, a frequency-selective reflecting surface (FSRS) or surfaces having transmissive reflective characteristics which depend on the frequency.
As one of such FSRSs, there is known a metallic plate having square apertures periodically arranged in a lattice form. This lattice apparently serves as an inductance in a relatively low frequency region, and its transmission is 1 in principle at its resonance frequency. In a higher frequency region, there arise higher modes, each having its own resonance frequency and a certain transmission smaller than 1.
There is known a technique by which a plurality of such lattices are used in a lower frequency region, i.e., the region where the lattices act as inductances, to separate frequencies by utilizing the interaction resonance resulting from interactions between the lattices. This prior art, however, has the disadvantage that its resonance characteristic curve is steeply inclined and, if a wide band pass characteristic is to be obtained, will require many lattices, which not only are uneconomical but also increase transmission losses.
To obviate this disadvantage, the present inventors previously proposed a frequency separator whose pass band is set in a frequency region higher than the region where an FSRS having a lattice of square apertures is considered an inductance but lower than the inherent resonance frequency of the lattice and in which a plurality of lattices are arranged at prescribed intervals. Reference is made to the published unexamined Japanese patent application No. 137703/81. Lattices in the pass band so set can be regarded as resonance elements of inductance capacitances (LCs), and the resonance of each lattice coupled with that resulting from interactions between the lattices enabled a frequency separator having a wide band pass characteristic to be realized.
This frequency separator proposed by the present inventors, however, involves the problem that, because it uses a lattice of square apertures, incoming electromagnetic waves of the transverse electric (TE) mode and those of the transverse magnetic (TM) mode will have different resonance frequencies if those waves obliquely come incident on an FSRS. This results in a deterioration in its frequency characteristic and leads to the frequency characteristic widely different from that for normally incident waves. In connection with this problem, there is known a technique using a lattice of rectangular, instead of square, apertures. It is disclosed in, for example, "A Quasi-Optical Polarization-Independent Diplexer for Use in the Beam Feed System of Millimeter-Wave Antennas" by A. A. M. Saleh et al published in the IEEE Transactions on Antennas and Propagation, Vol. AP-24, No. 6, November 1976, pp. 780-785. According to this article, the periodicity and size of apertures in the lattice are so determined that, the FSRS being regarded as an inductance, the inductance of the vertical strip of apertures and that of the horizontal strip be identical with respect to obliquely incident waves. However, this proposal, which regards the lattice as an inductance, cannot be helpful in improving the performance of a frequency separator like that proposed by the present inventors, in which the lattice is caused to serve as an LC resonance element with a view to giving the separator wide band pass characteristics.